Method of making stainless steel containing borides

ABSTRACT

A method for making stainless steel containing small, uniformly distributed boride particles and the product of such process having an improved absorption cross-section for thermal neutrons, in which a stainless steel containing about 0.1 to 4 percent by weight boron after being prepared in a convenient intermediate form is heated at or just above a critical temperature of from 2,275* to 2,325* F. at least long enough for it to be heated throughout to temperature and then it is rapidly cooled. The intermediate form is then worked to effect substantially uniform distribution of the boride particles. When necessary to prevent sagging and tearing of the intermediate form while it is being heat treated, it is supported while at heat. After cooling the support is removed before working.

[ Mar. 19, 1974 United States Patent .1 1

Bendel METHOD OF MAKING STAINLESS STEEL 3.235.417 2/1966 Roy et a1.148/136 ONT N BORIDES 11/1967 Foster 61! a1. 75/123 F [75] Inventor: LeeP. Bendel, Lebanon, NJ. Primary E mminer w' W stauard 73 Attorney,Agent, or Firm-Edgar N. Jay

Assignee: Carpenter Technology Corporation,

Reading, Pa.

[57] ABSTRACT A method for making stainless steel containing small,uniformly distributed boride particles and the of such process having animproved absor section 22 Filed: Mar. 10, 1970 1211 Appl. No.: 18,045

product n crossptio 52 us. 148/12, 148/123, 148/12.4 thermal neutmns'whch a Stamless Steel containing about 0.1 to 4 percent by weight boronInt. Cl. C21d 7/00, C21d 7/14 after being prepared in a convenientintermediate form is heated at or just above a critical tern of from 2perature F. at least long enough for it to temperature and then it is F0o 2 ,1 4/ 2H x 1 M 2m 1 00 4 1 w r. a e S f 0 d 1 .P F 1 8 5 ReferencesCited to be heated throughout UNITED STATES PATENTS rapidly cooled. Theintermediate form is then worked to effect substantially uniformdistribution of the boride particles. When necessary to prevent satearing of treated 3.415.640 Lambert............................ 75/.5AC gging and 3.598.567 the intermediate form while it is being heat2314-563 it is supported while at heat. After cooling the 3965968support is removed before working.

8/1971 11/1957 Dyrkacz et al.. 11/1962 10/1964 Dyrkacz et 3.152.934 Lulaet 3.192.040

9 Claims, 4 Drawing Figures 6/1965 Goda et 75/128 F PATENTED MR 1 9 I974SHEEI 1 [IF 2 FlG.l

FIG.2

PATENIEUHARISISM MI 2 BF 2 3,798,075

FIG.3

FIG.4

METHOD OF MAKING STAINLESS STEEL CONTAINING BORIDES BACKGROUND OF THEINVENTION This invention relates to a method of making a stainless steelalloy, and, more particularly, to a method for making a stainless steelalloy containing substantial amounts of boron having an improvedabsorption cross-section for thermal neutrons, and the product of suchprocess. It is to be understood that here and throughout thisspecification and claims, the term boron" when it appears alone is usedin its generic sense to include naturally occurring boron (which usuallycontains about 18 percent boron-10), natural boron enriched withboron-l0, or boron-10. The present invention is applicable to stainlesssteels containing any one or more of those forms of boron.

It has hitherto been known that naturally occurring boron is a desirableaddition to stainless steel for use in the fabrication of nuclearreactor control rods because it has a favorable absorption cross-sectionfor thermal neutrons. Its isotopic form boron-l has a substantiallyhigher absorption cross-section. Control rods have been made ofaustenitic stainless steel containing about 0.1 to 2.0 percent boron,but they left much to be desired whatever the form of the boron becauseof the difficulty hitherto experienced in ensuring the formation ofsmall enough boride particles.

In practice, when control rods containing boron are in use in a nuclearreactor, thermal neutrons effect a transmutation of boron-l0 to helium.To the extent that the boron-l0 present in the alloy is converted tohelium, the control rod is weakened, and the useful life of such controlrods depends upon the distribution of boron-1O in the alloy. The moreuniform the distribution of a given amount of boron-l0, the less adverseis the effect of such helium formation.

Usually, boron, when present in stainless steel, is in the form ofborides having a more-or-less complex structure, depending upon thecomposition of the steel. In the case of a chromium-nickel stainlesssteel, particles comprising M- B are formed in the steel matrix with Mequal to varying amounts of the elements Fe, Ni and Cr. While the sizeof such particles alone does not determine the degree to which thedistribution of the boron departs from the desired uniformity, it isevident that the larger the size of such particles, the more boron eachcontains, and the less uniform is the distribution of the boron in thesteel alloy. That is to say, when the borides are large, the boron issegregated even though the particles themselves may be uniformlydistributed in the steel alloy. And, as a result of such segregation,the formation of helium during exposure of the steel to thermal neutronshas a greater weakening effect because the voids formed by thetransformation are larger than those formed from the smaller particles.

In recognition of the desirability of more uniform boron distribution insuch materials, the users thereof have required that theboron-containing particles be small. less than a calculated length ofabout 5 microns as determined by the intercept method of countingborides in stainless steel. However, in practice, I have found that thesize of the boride particles could not be controlled by means ofconventional manufacturing practices to provide the desired smallboron-containing particle size.

SUMMARY OF THE INVENTION It is therefore a principal object of myinvention to provide a method for making stainless steel containingboron in which the boron is more uniformly distributed than hitherto.

Another object is to provide a process which ensures the attainment ofboride particle sizes consistently less than about 5 microns long oreven less than about 2 microns long as calculated from interceptcounting of borides in stainless steel. with the particles substantiallyuniformly distributed throughout the matrix of the steel.

I have discovered that when the stainless steel containing relativelylarge boride particles is heated to a critical temperature, the boridesundergo a high temperature reaction such that when the steel is rapidlycooled from that temperature, very fine boride particles are produced.Then the steel is hot and/or cold worked to the size required forfabrication into the f desired-end products, and the working servesuniformly to distribute the fine boride particles.

The process of the present invention is preferably carried out bystarting with the stainless steel alloy in an intermediate form such asa billet of convenient size. The temperature to which the intermediateform must be heated throughout for the required reaction to take placeis sharply critical. The minimum temperature required can vary inpractice from about 2,275 F. to 2,325 F. depending upon such variablesas the composition of the steel and the accuracy of the temperaturemeasuring equipment used. But in practice, the required temperature isreadily determined as will be more fully pointed out hereinbelow.

Care must be exercised when the steel is subjected to the hightemperature treatment that the intermediate form does not sag or tear tosuch an extent as to be unsuitable for hot or cold working to a finishedshape. When necessary to prevent such damage, the form can be supportedin a suitable tray or by means of a canister which is removed before theintermediate form is worked.

BRIEF DESCRIPTION OF THE DRAWING Further objects and advantages of thepresent invention will be apparent from the following detaileddescription thereof and the accompanying drawing in which FIG. 1 is aview of a photomicrograph showing the microstructure of a stainlesssteel alloy containing 1 percent boron under a magnification of 500times treated in accordance with the present invention;

FIG. 2 is a similar view at the same magnification of the same type ofalloy but with 0.37 percent boron at an intermediate stage of the methodof the present invention;

FIG. 3 is a similar view at the same magnification showing themicrostructure of the alloy shown in FIG. 2 after completion of themethod of the present invention; and

- FIG. 4 is a similar view at the same magnification showing themicrostructure of the alloy having the analysis shown in FIG. 2 butwithout the benefit of the treatment in accordance with the method ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the presentinvention, a stainless steel alloy is prepared comprising in weightpercent within the tolerances of good commercial melting practices:

Percent Carbon up to 0.25 Manganese up to 10 Silicon up to 2 Chromium12-26 Nickel up to 22 Molybdenum up to 4 Copper up to 4 Aluminum up to 4Titanium up to 1.25 Columbium up to 1.25 Nitrogen up to 0.7 Boron 01-4the balance iron except for incidental impurities and such otherelements as may be desired which do not detract from desired propertiesor objectionably interefere with the formation and substantially uniformdistribution of fine borides. For example, if better freemachiningproperties are desired up to about 1 percent of the customarily usedfree-machining additives can be included. In this category, I include upto about 1 percent of one or more of the elements phosphorus, sulfur,selenium and tellurium.

In carrying out the present invention, a steel of the desiredcomposition within the foregoing broad range is prepared utilizingconventional melting and casting techniques. In general, the compositionof the steel and the shape and properties required in the products to beformed therefrom will dictate whether the alloy should be prepared usingair melting techniques or whether a controlled atmosphere should beused. It can also be readily determined by those skilled in the artwhether the alloy is to be melted and cast as ingots which are thenworked into intermediate forms or whether such intermediate forms are tobe directly cast from the melt. For the production of control rodscontaining boron for use in nuclear reactors, I prefer to melt the steelin a vacuum-induction furnace and then cast the melt into ingots whichare in turn hot worked to a suitably sized billet as the intermediateform. Up to this point my process does not differ in any way fromconventional practices.

Then, in accordance with my invention the intermediate form of thesteel, which can be a billet as was seen, is heated to 2,275 F. to 2,325F. or just above long enough for the billet or other intermediate formto be heated throughout. Then the billet is rapidly cooled.

This results in the formation of extremely small boride particles,calculated as being less than 5 microns long. In practice, particlesizes calculated to be about 2 microns or less can be consistentlyprovided. Now the thus treated intermediate form is processed by hotand- /or cold working to the finished form using conventional workingpractices. The boride particles retain the small size. but may becomespheroidized and are uniformly distributed.

The mechanism by which such small boride particles characteristic of thepresent invention are formed is not fully understood. However, though Ido not wish to be bound thereby, I believe at this time that a eutecticreaction takes place and thereafter when the steel is rapidly cooled theextremely small boride particles are formed.

LII

LII

It is to be noted that unless the temperature is carefully controlledand/or, depending upon the size and shape of the intermediate form, theform may not be fully self-supporting at the treating temperature. Inthat event, the intermediate form is supported mechanically while it isat the treating temperature. For this purpose, a tray or other suitablesupport means can be used. A preferred form of support is provided whenthe intermediate form is enclosed in a canister formed of a materialsuch as iron or another steel which remains solid and self-supporting attemperatures somewhat above the treating temperature. The shape of thecanister should conform closely enough to that of the intermediate formso as to be able to support the latter at the elevated temperature used.The canister and the form within it are then heated to at least about2,275 F. to 2,325 F and the whole is maintained at that temperatureuntil the intermediate form is heated throughout to that temperature.Then the assembly is rapidly cooled, and the canister is removed bychemical or mechanical means. As before, the intermediate form is thenhot and/or cold worked to the desired finished form.

EXAMPLE NO. 1

As an example of my invention, a billet was prepared by hot working froman ingot, formed in an air induction furnace, containing 0.039 percentcarbon, 1.69 percent manganese, 0.83 percent silicon, 0.016 percentphosphorus, 0.006 percent sulfur, 18.77 percent chromium, 15.11 percentnickel, 0.20 percent molybdenum, 0.17 percent copper, 1.00 percentboron, and the balance iron except for incidental impurities.

The thus formed billet after surface preparation, e.g. machining, wasentirely enclosed in a canister formed of A.I.S.I. Type 304 stainlesssteel, and the assembly was heated at a temperature of about 2,3l0 F.for 1 hour which was long enough for all the billet, about a 1.7 inchround in transverse cross-section, to be brought to that temperaturethroughout. Whereupon the assembly was rapidly cooled by quenching inwater, and then the canister was removed from the billet by machining.In this condition, the boride particles have the desired smallcalculated size of less than 5 microns but are not usually uniformlydistributed as is also required.

The billet was then forged to 1% inch square from a furnace temperatureof 2,100" F. followed by hot rolling to a 7/16 inch round corneredsquare rod, also from a starting temperature of 2,100 F. Then, by coldrolling, the 7/16 inch round cornered square rod was then reduced to3/16 inch strip. In FIG. 1 there is shown a photomicrograph of thestructure of a portion of the resulting strip at a magnification of 500times which can be compared to the structure of similar material treatedin the conventional way and illustrated in FIG. 4 yet to be described indetail hereinbelow. Using a standard metallographic inspection procedurefor intercept counting of borides, the boride size of Example 1 as shownin FIG. 1 was calculated to be less than 1.79 microns in radius.

EXAMPLE NO. 2

As a further example of my invention, a billet was prepared and treatedas was described in connection with Example 1 except as follows. Thebillet had an analysis containing 0.035 percent carbon, 1.73 percentmanganese, 0.53 percent silicon, 0.013 percent phosphorus, 0.008 percentsulfur, 18.57 percent chromium, 14.11 percent nickel, 0.11 percentmolybdenum, 0.05 percent copper, 0.37 percent boron, and the balanceiron except for incidental impurities. The microstructure of the billetat this stage showing the undesirably larger borides is shown in FIG. 4at a magnification of 500 times. The billet was 1 in. X 2 in. X 6 in.,was not enclosed in a canister and no special support was provided, andwas heat treated at about 2,300 F. for 1 hour followed by coolingrapidly by quenching in water. The resulting microstructure as shown inFIG. 2 at a magnification of 500 times is seen to contain the desirablysmall boride particles, but without the effects of working which bringabout the desired more uniform distribution and also tend to spheroidizethe particles.

Following the step of quenching in water, the billet was hot worked toone-fourth inch thick from a furnace temperature of 2,l00 F. and thencold rolled to 0.038 inch thick strip. A photomicrograph was preparedshowing the resulting structure, also at a magnification of 500 times,and is shown in FIG. 3.

A comparision of FIGS. 2, 3 and 4 clearly shows the significantreduction in the size and improved distribution of the boride particlesprovided by the present invention. Though no special precautions need tobe taken in carrying out my process, it is to be noted that thetemperature at which the heat treatment is carried out is critical andrelatively narrow. For any given composition the optimum temperature isreadily determined. Test specimens of the desired composition are heatedlong enough for the high temperature reaction to take place at selectedtemperatures until substantially the minimum temperature for thereaction is found. Then an upper limit is determined by examining theeffects of high temperatures on different specimens. For example, in thecase of the composition of Examples 1 and 2, it was found that the heattreatment had to be carried out between about 2,300 F. and about 2,340F. because at lower temperatures below about 2,275-2,300 F. the desiredreaction did not occur and above about 2,340-2,365 F. the formation ofshrinkage voids became objectionable and resulted in unsound material.When l-inch cube specimens having the same composition as the billet ofExample 2 were heat treated at about 2,200 F. for l, 4 and 8 hours,there was no apparent effect upon the size of the boride particles.

While a wide variety of compositions falling within the broad rangehereinabove stated can be used in carrying out the process of myinvention, the compositions of Example 1 and 2 are illustrative of mypreferred range which consists essentially of, in weight percent, up toabout 0.03 to 0.08 percent carbon, up to about 2 percent manganese, upto 0.045 percent phosphorus, up to 0.03 percent sulfur, up to about 1percent silicon, 17 to 20 percent chromium, 7 to percent nickel, 0.1 to2 percent boron and the remainder iron except for incidental impurities.Within that range, nickel is included in an amount of at least about 12percent when its effect on corrosion resistance and other properties isdesired.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

I claim:

1. In the method for making stainless steel articles containingsubstantially uniformly distributed fine boride particles, the steps ofmaking an intermediate form comprising in weight percent about Carbon upto 0.25 Manganese up to 10 Silicon up to 2 Chromium 12-26 Nickel up to22 Molybdenum up to 4 Copper up to 4 Aluminum up to 4 Titanium up to1.25 Columbium up to 1.25 Nitrogen up to 0.7 Boron 0.1-4 Phosphorus)Sulfur Selenium up to 1 Tellurium and the balance substantially iron,then heating the intermediate form to at least about 2,275 F. to 2,325?F. for a time at least long enough for said intermediate form to reachat least said temperature substantially throughout, then rapidly coolingsaid intermediate form to produce fine boride particles, and thenworking said intermediate form to effect substantially uniformdistribution of the fine boride particles therein.

2. The method set forth in claim 1 in which the longest dimension ofsaid boride particles as calculated by intercept counting is less thanabout 5 microns.

3. The method set forth in claim 1 in which the intermediate form issupported while being heated to prevent sagging and tearing.

4. The method set forth in claim 1 in which said intermediate form priorto heating is enclosed in a canister formed of a material which cansupport said intermediate form therein during said heating.

5. The method set forth in claim 4 in which said intermediate form isremoved from said canister after cooling and before said working.

6. The method set forth in claim 5 in which the stainless steel articlehas an essentially austenitic microstructure and comprises in weightpercent about Carbon up to 0.08 Manganese up to 2 Silicon up to 1Phosphorus up to 0.045 Sulfur up to 0.03 Chromium 17-20 Nickel 7-15Boron 0.1-4

and the balance substantially iron.

7. The method set forth in claim 6 in which the 1ongest dimension ofsaid boride particles as calculated by intercept counting is less thanabout 5 microns.

8. The method set forth in claim 6 which comprises at least about 12percent nickel and no more than about 2 percent boron.

9. The method set forth in claim 8 in which said assembly is heated to atemperature no higher than about 2,340 F. to 2,365 F.

2. The method set forth in claim 1 in which the longest dimension ofsaid boride particles as calculated by intercept counting is less thanabout 5 microns.
 3. The method sEt forth in claim 1 in which theintermediate form is supported while being heated to prevent sagging andtearing.
 4. The method set forth in claim 1 in which said intermediateform prior to heating is enclosed in a canister formed of a materialwhich can support said intermediate form therein during said heating. 5.The method set forth in claim 4 in which said intermediate form isremoved from said canister after cooling and before said working.
 6. Themethod set forth in claim 5 in which the stainless steel article has anessentially austenitic microstructure and comprises in weight percentabout Carbon up to 0.08 Manganese up to 2 Silicon up to 1 Phosphorus upto 0.045 Sulfur up to 0.03 Chromium 17-20 Nickel 7-15 Boron 0.1-4 andthe balance substantially iron.
 7. The method set forth in claim 6 inwhich the longest dimension of said boride particles as calculated byintercept counting is less than about 5 microns.
 8. The method set forthin claim 6 which comprises at least about 12 percent nickel and no morethan about 2 percent boron.
 9. The method set forth in claim 8 in whichsaid assembly is heated to a temperature no higher than about 2,340* F.to 2,365* F.